1. Field of the Invention
This invention relates to a dot-etching solution for dot-etching a halftone dot image of a metal composed mainly of aluminum.
2. Description of the Prior Art
A method for forming a metal image has heretofore been known which comprises imagewise exposing to electromagnetic radiation a metal image-forming material consisting of a support such as a glass plate or a synthetic resin film, a thin layer of metal composed mainly of aluminum on the support and a photosensitive resin (or photoresist) layer on top of the metal layer, dissolving the exposed or unexposed portion of the photosensitive resin layer with an organic solvent to remove that portion, and dissolving the uncovered metal layer beneath that portion using an etchant solution thereby to form an image composed of the metal layer and the photosensitive resin layer, or if desired, to form an image composed only of the metal layer by removing the photosensitive resin layer. If electromagnetic radiation is used to form a halftone dot image in accordance with this method, a metal image corresponding to the halftone dots can be obtained. In order to vary the density of the halftone dot image, it has been the practice in the art of handling halftone images to perform an operation called "dot-etching" whereby the size of the dots is partially reduced.
Usually, lithographic films are used to produce halftone images. In the dot size reduction of lithographic films with a reducing solution, the reducing solution acts on the entire surfaces of the silver grains that form the image. Hence, this reducing operation frequently results in a decrease of the density of the individual dots simultaneously with the reduction of the halftone dot area. In contrast, since in the above-described image-forming material, a resin layer substantially impermeable to a dot-etching solution is present on top of a thin metal layer which is the main material for forming an image by not transmitting light, this type of image-forming material has the advantage that the surfaces of the halftone dots are not etched and since etching from the sides of the dots only occurs the dot-etching results only in the reduction of the area of the dots. On the other hand, the metal must be etched by the dot-etching solution which has passed through a very small space between the substrate and the hardened photoresist layer on the dot of the metal layer. Hence, such a dot-etching solution must perform etching at commercially sufficient speeds. Moreover, such a solution should not degrade the photosensitive resin layer remaining on the metal layer nor contain substances harmful to the human body, and should reduce only the sizes of dots while correspondence of the shape of the etched dots to the shape of the dots before dot-etching is maintained.
Conventional reducing solutions comprising sulfuric acid, hydrochloric acid, nitric acid or hydrofluoric acid used for etching aluminum or aluminum alloys are unsuitable as dot-etching solutions for image-forming materials of the type described above. A solution consisting mainly of sulfuric acid has a slow etching speed, and long periods of time are required to achieve the desired amount of dot-etching. A solution consisting mainly of hydrochloric acid is hazardous to working personnel because hydrogen chlroide diffuses into the air, as described, for example, in Japanese patent application (OPI) No. 10737/75. A solution consisting mainly of nitric acid does not uniformly etch the thin metal layer, and results in etched dots having poor shapes. A solution consisting mainly of hydrofluoric acid is dangerous to handle.
Japanese patent publication No. 25611/71 discloses a system composed mainly of phosphoric acid in which the phosphoric acid provides the main etching action of the surface of aluminum or an aluminum alloy, and a chloride or nitrate of a metal having a lower ionization tendency that aluminum, such as NiCl.sub.2 or Cu(NO.sub.3).sub.2, acts to promote the speed of etching. However, the use of nickel or copper in a treating solution to be discharged as waste should absolutely be avoided to protect against environmental pollution.